A Five-Level Inverter Topology With Extended Linear Modulation Range Till Full Base Speed Irrespective of Load Power Factor

In conventional inverters, the linear modulation range is limited to a maximum voltage space vector radius of 0.866 times the dc link voltage (V-dc), which corresponds to a maximum peak fundamental phase voltage of 0.577 V-dc. Beyond this, the inverter enters the overmodulation region, where the operation transitions to a six-step mode causing the voltage space vector to trace a hexagonal locus. This results in phase voltage waveforms with significant lower order harmonic content leading to torque ripples in the motor shaft and undesired harmonic losses. Thus, the achievable speed range of the induction motor in the linear modulation range is effectively limited to 90.5% of its full base speed. In contrast, the proposed five-level inverter topology generates a circular voltage space vector locus with a radius of up to 0.955 V-dc resulting in a maximum peak fundamental phase voltage of 0.637 V-dc. Thus, the linear modulation range is extended to full base speed and is independent of the load power factor. The proposed power circuit is based on two standard three-level neutral point clamped inverters and can be realized by two readily available three-level NPC inverter IC packages. Standard space vector pulsewidth modulation techniques can be employed for switching pulse generation.